Dynamics simulation of the effect of cosolvent on the solubility and tackifying behavior of PDMS tackifier in supercritical CO2 fracturing fluid

Due to the low viscosity of supercritical carbon dioxide (SC-CO2) fracturing fluid, the method of adding tackifier is generally chosen to improve its sand carrying ability. Recently, siloxane-based tackifier has been widely utilized because of low cohesive energy and good viscosity building properties, but cosolvents are somehow needed to be added in the system to improve its solubility in SC-CO2. In this study, polydimethylsiloxane (PDMS)- a widely used siloxane polymer- was selected as the object tackifier. First, the effects of the addition of cosolvents such as ethanol, acetic acid, cyclohexane and toluene on dissolution behaviors of PDMS in SC-CO2 fracturing fluid were investigated by molecular dynamics simulation. Moreover, the solubilization effects of polar and nonpolar cosolvents on PDMS in SC-CO2 fracturing fluid were studied by comparing the binding energy, cohesive energy density and radial distribution function between solvent-solvent and between solvent-solute. The simulation results showed that cyclohexane could improve the solubility parameters of SC-CO2 the most at the same cosolvent concentration, and SC-CO2 was more uniformly distributed in the system. Meanwhile, the interactions between toluene and SC-CO2 was the strongest, and SC-CO2 was generally uniformly distributed in the system. However, ethanol and acetic acid performed poorly because they could produce hydrogen bonds with certain directionality and saturation. It was revealed that the mechanism of using cosolvents to improve the solubility of PDMS in SC-CO2 was based on the equilibrium of intermolecular forces between CO2 and PDMS, between CO2 and cosolvent, and between PDMS and cosolvent. When the siloxane-based tackifier was a nonpolar material,cyclohexane was recommended as the cosolvent. If the siloxane-based tackifier itself had some weak polarity, toluene was more suitable.